I changed the IP Address of a domain hosted on a Plesk server and, after a couple of hours, everything seemed to point to the right place and the site was loading fine. When I woke up this morning, however, things seem to be pointing back to the original IP on my laptop (but it's still loading correctly on my phone). It's been a full 24 hours since I made the change. I'm just confused because it was working fine for half a day, and now on some connections seems to have reverted back to the old IP.

What are the most logical next steps for troubleshooting this? The DNS records all point to the new IP (via the A Record). Is this just a weird propagation issue with my ISP (meaning I should wait it out) or is there anything I can do to investigate?

If all the pubic DNS servers you tested are still pointing to the new IP, except your ISP, then your configuration is correct. Otherwise you have to go back and check your dns setup in Plesk, and confirm Apache is using that IP too.

You could be hitting a caching nameserver that still has the old record. I've seen that happen a fair number of times. If it doesn't clear up soon, then there may be an issue. A good way to see if the problem is just DNS caching or something locally is to ping the site at just-ping.com. That will show you where it's resolving in different places all through the world. By now, almost all of them should be going to the new IP. If they're not, then there's likely some problem with your host's nameservers or your zone file.

It's always a good idea to flush your DNS cache, as well. (Of course, this only flushes your computer's DNS cache and won't affect any sort of nameserver caching going on between you and your host's nameservers.)

Also, another good troubleshooting step is to see what your host's nameservers think the domain is pointed. This will pretty much tell you exactly if it's a problem on that end or somewhere else: (local nameserver, caching, etc):